This application claims benefit of Japanese Patent Application No. 11-121715, filed on Apr. 28, 1999, and Japanese Patent Application No. 11-128469, filed on May 10, 1999, the entire contents of the Application Nos. 11-121715 and 11-128469 being incorporated by reference.
The present invention relates to an optical pickup in which an optical element having a function for increasing the numerical aperture, for example, a SIL (Solid Immersion Lens) or the like, is disposed close to a recording medium to perform recording and/or reproducing of information on the recording medium at a high density.
Japanese Patent Publication No. 2,553,275 discloses an optical system which is capable of performing recording and/or reproducing of information on a recording medium. In this conventional optical system, converging light from an objective lens is further converged through a semi-spherical SIL so that a diameter of a spot formed on a recording layer of the recording medium is reduced. The semi-spherical SIL increases the numerical aperture of the optical system by a multiple corresponding to its own refractive index.
In an information recording and reproducing technique of the optical system, when information is recorded on the recording layer of the recording medium and it is reproduced from the recording layer, it is necessary to put the semi-spherical SIL in much close to the recording layer. The distance from the semi-spherical SIL to the recording layer is significantly small as compared with the distance from an objective lens to the recording layer in a conventional information recording and reproducing technique of another kind in which light converged by only the objective lens is projected through a front side surface of such a recording medium as a CD (Compact Disc) or a MO (Magneto-Optical Disc) onto a recording layer on a back side surface thereof to perform recording and reproducing of information on the recording layer. For this reason, the recording and reproducing technique using the SIL is called as a near field recording and reproducing technique, while the recording and reproducing technique using only the objective lens is called as a far field recording and reproducing technique.
Nikkei Electronics, Pages 99 to 105, issued on Jun. 16, 1997 (No. 691) has disclosed improvement for such a near field recording and reproducing technique. In the improved near field recording and reproducing technique, a super semi-spherical SIL or a stigmatic focusing SIL is used instead of the semi-spherical SIL. The super semi-spherical SIL or the stigmatic focusing SIL increases the numerical aperture of an optical system using the same by a multiple equal to the second power of the refractive index thereof.
Japanese Patent Application KOKAI Publication No. 8-315404 has disclosed an optical pickup having an optical system in which an objective lens and a generally semi-spherical SIL are combined. The optical pickup records information on and reproduces it from a recording surface in a state where the generally semi-spherical SIL is brought in much close to a thin cover glass which covers the recording surface of the recording medium.
In the plurality of the conventional techniques described above, when recording and/or reproducing of information is performed on the recording layer of the recording medium by using the SIL, it is necessary to bring the SIL in much close to the recording layer. In a typical example, the SIL is brought in close to the recording layer up to at least 500 mxcexc or less, or 200 nm or less. Incidentally, the numerical aperture increased by a lens with a high numerical aperture is at least 0.65 or more. In recent years, a technique for increasing the numerical aperture to 0.8 to 0.9 or more has been published in relevant academic meetings.
However, in the above-mentioned conventional near field recording and reproducing techniques, there are the following problems.
Though the recording layer on the front side surface of the recording medium is covered with a thin transparent protective film, contaminant such as moisture, organic materials, dusts and the like in the atmosphere adheres to the front side surface of the protective film. Accordingly, when the recording medium is rotated in order to perform recording and/or reproducing of information on the recording layer on the front side surface of the recording medium in a state where the SIL has been brought in much close to the recording surface on the front side surface of the recording medium, contaminant is scattered by a centrifugal force acting thereon and the contaminant is adhered and deposited on the SIL. Particularly, the contaminant which has been scattered from the protective film of the recording medium is adhered and deposited on a facing surface of the SIL facing the protective film. When the adhesion and deposition of such contaminant on to an optically effective area on the facing surface of the SIL become heavy, the transmittance of laser beam on the optically effective area is gradually decreased (the optical performance of the SIL is gradually lowered), so that it becomes difficult to record information on and/or reproduce it from the recording surface of the front side surface of the recording medium for a long time of period.
The present invention has been derived from these circumstances, and an object of the present invention is to provide an optical pickup which prevents an optical performance of an optical element, such as a lens with a high numerical aperture, including a SIL, and disposed close to a recording layer of a recording medium from lowering due to adhesion and deposition of contaminant onto the optical element even under use for a long period of time, and which can stably perform recording and/or reproducing of information on a recording medium for a long period of time.
In order to achieve the object of the present invention, an optical pickup according to the present invention comprises: a light source for emitting laser beam; an optical system for converging the laser beam from the light source; and an optical element having a function for increasing a numerical aperture of the optical system, wherein recording and/or reproducing of information on a recording medium is performed by using the laser beam from the light source while the optical element is brought in contact with or brought in close to the recording medium, and the optical element has a contamination-preventing structure.
In the optical pickup according to the present invention and structured as described above, the contamination-preventing structure of the optical element may include a water-repellent portion which is provided on at least an optically effective area in a surface portion, facing the recording medium, in the optical element and prevents contaminant from adhering and depositing on the at least optically effective area, it may include a contaminant attracting portion which is provided on an area except the optically effective area in the surface portion, facing the recording medium, in the optical element and attracts contaminant, or it may include a projection which is provided on at least the optically effective area in the surface portion, facing the recording medium, in the optical element and increases flow rate and pressure of air flowing between the recording medium and the projection.
The water-repellent portion may have a coating of a predetermined water-repellent material, and may have an ion implanted layer formed by implantation of predetermined ions.
The predetermined water-repellent material may be inorganic material or organic material.
Also, the predetermined ion may be selected from one or more of at least nitrogen, carbon, fluorine, and silicon.
Furthermore, the optical element is a solid immersion lens (SIL).
The contaminant attracting portion may include a rough surface which is rougher than the optically effective area in the surface portion, facing the recording medium, of the optical element or a plurality of grooves, and the rough surface may be formed by performing chemical etching or physical etching on the area, except the optically effective area, in the surface portion, facing the recording medium, of the optical element.
The contaminant attracting portion may be combined with a water-repellent portion provided at the optically effective area in the surface portion, facing the recording medium, of the optical element.
The contaminant attracting portion may include a thin film of metallic oxide.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.